The present invention relates to dewatering of aqueous slurries of solid coal particulates and more particularly to an improved filtration process and dewatering aids therefor.
The vast majority of mineral processing methods require that the valuable mineral constituent(s) be separated in aqueous slurry form. After separation, the various mineral values must be dewatered to enable further processing and/or for recycle of the water for economical and/or ecological reasons. Further economic justification for dewatering includes lowering of transportation costs; lowering of energy costs during filtration; and elimination of freezing of the concentrates in railroad cars, barges, silos, stockpiles, etc. Additionally, chemical dewatering in filters can range from about 3 to 25 times less expensive than thermal drying.
Most commercial liquid/solid separation is effected by a rotary (drum or disk) vacuum filtration system or by centrifugal dryers. Filtration often is insufficient to meet the residual water specification and must be supplemented by thermal drying. Drum filters have been described in the literature as a rotating drum covered with filter cloth which rotates with its lower portion immersed in a continuously fed bowl with vacuum being applied beneath the cloth causing solids to deposit as a cake while filtrate passes therethrough. As the drum rotates, the filter cake leaves the slurry and air is drawn therethrough in a dewatering step, often known as the drying step. Dewatering aids are known to be effective in such drying step of the process. Finally, the dewatered filter cake is discharged by a variety of techniques including blowing action, knives, belts, strings, or rollers. Pearse, et al., "The Use of Flocculants and Surfactants in the Filtration of Mineral Slurries", Filtration Separation, January/February, 1983.
Many variables influence filtration efficiency. For example, the cake properties have been reported to be a function of particle size and size distribution, shape, packing, and dimensions of the cake. Fluid properties have been reported to be a function of density and viscosity while interfacial properties have been reported to be a function of surface tension (gas/liquid) and interfacial tension (gas/liquid and gas/solid). Other reported variables include temperature and pressure gradient rate of displacement. Kirk-Othmer, Encyclopedia of Chemical Technology, 1984 Supplement, page 310 et seq. Any change in any one of the above variables results in an increase or decrease of the final moisture content of the recovered ore. For instance, by raising the temperature from 15.degree. C. to 80.degree. C., the water viscosity drops by a factor of three and, consequently, the flow rate of water through the cake theoretically is tripled. Occasionally, steam is used to increase filtration rate on drum filters based on such viscosity phenomenon.
In coal, for example, four different types of water have been reported. These types of water include bulk water, capillary water, surface water, and inherent moisture. Bulk water is the water phase in an ore particle slurry or suspension. Particles in close proximity define capillary voids which retain water, i.e. capillary water. A thin sheath of water surrounding a particle due to surface wetting and water adhesion is known as surface water. Finally, moisture is retained in the fine pore structure of the coal and is known as inherent moisture. Blubaugh, et al., "Dewatering Agents in Coal Preparation", Nalco Chemical Company, Reprint 30.
Two distinct classes of chemicals are available for improving filtration properties of mineral ore slurries. The first class comprises flocculant "filter aids". These compounds are mostly linear, long-chain, water-soluble anionic or nonionic polymers based on polyacrylamide. The polymers bridge individual fine particles giving multi-particle aggregates. The aggregates have greater permeability, allowing for faster flow of water through the cake. These larger aggregates also assist in preventing filter cloth blinding by extremely fine particles. The second reported class of chemical filtration additives comprise surfactant "dewatering aids". The surfactants most often are heteropolar molecules composed of hydrophylic and hydrophobic groups. Commercially, sulfosuccinates are the most widely used surfactant dewatering aid. U.S. Pat. No. 4,156,649 reports the use of ethoxylated linear or branched alcohols as surfactant dewatering acids. U.S. Pat. Nos. 4,206,063 and 4,207,186 add a C.sub.8 -C.sub.18 hydrophobic alcohol to an ethoxylated linear branched alcohol as a dewatering aid.
Theories of surfactant action in dewatering contexts include the surfactants concentration at the liquid/air interface resulting in reduction of surface tension, thus allowing filter cake capillaries to drain more readily; or their adsorption on solid surfaces with their hydrophobic group oriented towards the aqueous phase, thus rendering the ore particle surfaces more hydrophobic, i.e. more willing to shed the surface water. Pearse, et al., supra; Fall, et al., "Influence of Energy in Filter Cake Dewatering", Aufbereitungs-Technik, No. 3/1987, pp. 115-125; and Purdy, "Chemical Dewatering Aids for Mineral and Coal Slurries", Symposium Chemical Reagents in the Mineral Processing Industry, February, 1987. Both theories have validity and have been justified by results reported in the literature. However, each theory has its drawback and at various stages of the filtration process, each will have its dominant role. For example, the reduction of surface tension tends to be important at the onset of the dewatering cycle while increased hydrophobicity tends to dominate towards the end of the dewatering cycle.
Complicating the foregoing are additional factors, such as cracking of the filter cake, bubbles in the filter cake, etc. The elimination of these factors improve filtration efficiency. The thickness of the filter cake (pickup) influences not only the capacity but the resulting residual moisture. Surfactants can affect these variables positively or negatively.
In the dewatering of aqueous slurries of solid coal particles, a variety of amine compounds have been proposed in the art for use as dewatering aids (Brookes, "Zeta Potential Contact Angle and the Use of Amines in the Chemical Dewatering of Froth-Floated Coal", Powder Technology, 49 (1984) 207-214). Various amines proposed include, for example, oleyl amine, oleyl diamine, C.sub.15 -C.sub.18 primary amines, secondary C.sub.13 -C.sub.15 saturated amines, ethoxylated amines and diamines, and oleyl amine acetates. Purdy, "Chemical Dewatering Aid for Mineral and Coal Slurries", Chemical Reagents in the Mineral Processing Industry, Society of Mining Engineers, Inc., 1986 Symposium at Littleton, Colo., Chapter 28, states that he ". . . is not aware of cationic surfactants in commercial use in mineral filtration . . . " (at page 264). Curiously, however, U.S. Pat. No. 4,370,272 asserts that a specific dimethyl monoamido ester quaternary ammonium compound is suitable for use in the dewatering of coal. No data or other substantiation of this assertion is contained in the patent.